In the operation of modern internal combustion engines, it is desirable, and often necessary because of government regulation, to obtain maximum fuel economy. From a purely economic standpoint, with the increasing cost and, from time to time, shortages of hydrocarbon fuels, improving the fuel efficiency of internal combustion engines is very desirable. Accordingly, methods for improving fuel economy have been the subject of numerous U.S. Pat. Nos., including U.S. Pat. No. 4,584,115, 4,237,022, and U.S. Pat. No. 4,406,802.
Many polymeric materials have been developed for use as viscosity improvers. The purpose of viscosity improvers is to minimize the changes in viscosity of an oil composition when it is exposed to a variety of temperatures. Included among viscosity improvers known in the art are polyolefins, styrene-butadiene copolymers, high molecular weight polyesters, and the like. Examples of such products appear in U.S. Pat. No. 2,394,909, U.S. Pat. No. 3,598,738, U.S. Pat. No. 3,772,169, and U.S. Pat. No. 3,795,616.
Recently, efforts have been directed toward preparing additives for lubricants and fuels which have multifunctional properties. The advantages of such multifunctional additives are obvious. For example, it is often less costly to prepare a single additive having multifunctional characteristics, rather than preparing several additives, each having a single characteristic. When such multifunctional additives are available, it is not necessary to have a multitude of storage tanks to store a variety of additives for blending. Storage space requirements and shipping costs are often reduced when the multifunctional additive provides, on a reduced weight basis, the same performance as two or more additives which each provide a single function. Examples of well-known multifunctional additives are dispersant-viscosity improvers, basic metal salts which have antioxidant properties, and the like.
Certain polysuccinate esters are known. U.S. Pat. No. 2,993,773 describes numerous esters of alkenyl succinic acids and anhydrides, and the use thereof as deposit modifiers in fuels. Mixed esters of polyhydric alcohols and the use thereof as synthetic oils are described in U.S. Pat. No. 2,575,196. U.S. Pat. No. 2,134,736 refers to mono and polyhydric alcohol esters of polybasic carboxylic acids as additives for lubricating oils. There is no suggestion of polyesters. U.S. Pat. No. 2,394,909 refers to polyesters having average molecular weights between 5000 and 25,000 as viscosity improvers for lubricants. U.S. Pat. No. 2,561,232 describes diesters derived from dibasic acids and monohydric alcohols. U.S. Pat. No. 2,570,037 relates to diesters derived from dibasic acids and ether-alcohols. U.S. Pat. No. 2,929,786 describes synthetic lubricating oil compositions wherein a reaction product of a dibasic acid and a glycol is employed as the synthetic lubricant or as an additive for a synthetic lubricant. U.S. Pat. No. 3,381,022 describes polyesters which have a high molecular weight substituent on the succinic group. U.S. Pat. No. 4,209,411 describes polyesters derived from a hydrocarbon substituted succinic anhydride and a cyclic poly(methylol) compound.